1. Field of the Invention
This invention relates to apparatus for depositing layers of material on semiconductor substrates and, more particularly, to reducing the deposition of such material on the backside of the substrate.
2. Background
A layer of a material can be deposited on a substrate, such as a semiconductor wafer, using a chemical vapor deposition (CVD) or physical vapor deposition (PVD) process. An example of one such process is the deposition of tungsten on a silicon semiconductor wafer using a CVD process.
Prior to commencing the Tungsten CVD process, the wafer is loaded into a deposition chamber where it is mounted horizontally on a support, such as a susceptor, with a deposition surface facing upwards. Before being loaded into the chamber the upper deposition surface of the wafer is coated with a titanium nitride film. This is because tungsten does not readily adhere to the usual silicon dioxide (SiO.sub.2) surface of the wafer. It does, however, adhere to titanium nitride.
Once the coated wafer is loaded into the chamber, the deposition gas, usually tungsten fluoride (WF.sub.6), is introduced into the chamber via a gas inlet or "shower head" mounted above the wafer. Typically, the major part of the deposition process is carried out at elevated temperatures of between 250.degree. C. and 600.degree. C. and at sub-ambient pressures of 10.7 to 12 kPa (80 to 90 Torr).
In Tungsten CVD and other substrate processing operations, a primary goal is to obtain as many useful die as possible from each substrate. Many factors influence the ultimate yield of die from each substrate processed in these processing operations. These factors include the amount of contaminants that can attach to the substrate and contaminate its surface. Other factors include the processing variables which affect the uniformity and thickness of the layer of material deposited on the substrate of the substrate. Both of these and other factors must be carefully controlled in CVD and other processes to maximize the die yield from each substrate.
Unfortunately, CVD processing chambers include multiple sources of particle contaminants which, if received on the surface of the substrate, reduce its die yield. One source of particle contamination in CVD processing is the deposition of material on the edge and back/underside of the wafer. For a number of reasons, the deposition material layer does not firmly attach itself to the edge or back-/underside of the substrate and so the material layer deposited in these locations are known to flake off and become a particle contaminant both in the CVD and subsequent processing operations. This is particularly so in the tungsten CVD process if, as may be the case, titanium nitride has not been deposited on the edge and backside of the wafer. As described above, this means that the tungsten will not adhere firmly to these areas and will relatively easily flake off.
One method of controlling this unwanted edge and backside deposition is to use a shadow ring to reduce the occurrence of the deposition layer in these areas. A shadow ring is a masking member which usually rests on the substrate and covers its upper, outer, circumferential area. This limits access of the deposition gas to the contacted area of the substrate. As the shadow ring limits passage of process gas across the contacted circumferential area, it also limits access of the gas to the edge and backside of the substrate. The shadow ring, however, has not proven universally successful, due to wafer warpage and the volatile deposition gas still tends to migrate under the lip of the shadow ting and deposit unwanted material on the substrate edge and backside.
Apart from the existence of contaminants from sources such as unwanted deposition, processing variables also affect the die yield. One such processing variable (which affects the uniformity of the deposition material layer) is the uniformity of heating of the wafer.
In many prior art processing chambers the wafer is heated from below by means of heater lamps. These heater lamps heat, by means of infrared radiation, a susceptor on which the substrate is supported within the chamber. The susceptor in turn heats the substrate by conduction. One of the problems with this arrangement is that the susceptor (typically made of 8 millimeters thick aluminum with a ceramic support plate) has a relatively short lifespan. This means that it must be replaced, often as frequently as after 3,000 cycles, which results in increased labor costs and expensive downtime.
One alternative arrangement is to replace the heater lamps and thin susceptor with a heater pedestal for both supporting and heating the wafer. An example of this type of arrangement is described in copending U.S. patent applications 08/200,079 in the name of Lei, et al., and 08/200,862 in the name of Sinha, et al, both filed on Feb. 23, 1994 and assigned to Applied Materials, Inc. of Santa Clara, Calif. In this arrangement, the wafer is supported on a flat supporting surface of a heater pedestal mounted on a vertical stalk within the chamber. The pedestal is heated from within by means of a heating coil, and the wafer, in turn, is heated by the hot supporting pedestal. In order to provide for improved uniformity of heating of the wafer, this arrangement makes provision for drawing a "vacuum" at the interface between the underside of this wafer and the flat supporting surface of the pedestal. The resulting pressure differential across the wafer draws the wafer onto the pedestal resulting in improved uniformity of heating of the wafer.
As a result of this vacuum drawn at the backside of the wafer, however, processing gas can be drawn around the edge of the wafer and into the interface between the wafer and the pedestal. This can occur even in the presence of a shadow ring and results in unwanted edge and backside deposition even when a shadow ring is located over the wafer. As explained earlier, this unwanted deposition can lead to the generation of particle contaminants. Accordingly, the improved uniformity of heating is accompanied by the possible increase of unwanted edge and backside deposition.
The need exists, therefore, for an improved chemical deposition apparatus which more uniformly heats a substrate and, at the same time, deposits less material on the edge and backside of the substrate.